1. Field of the Invention
The present invention relates generally to inductors and, more particularly, to a system and method for producing an inductor with improved characteristics.
2. Description of the Related Art
FIGS. 7A and 7B illustrate a conventional layered-type inductor. This type of inductor is an example of a surface mounting type inductor. As illustrated in FIGS. 7A and 7B, the layered-type inductor is provided with a layered-type coil 52 formed by interconnecting a plurality of internal conductors 52a. The layered-type inductor also includes external electrodes 53a, 53b which are connected to respective end portions of the coil 52.
As illustrated in FIGS. 7A and 7B, such a layered-type inductor is commonly produced by laminating a plurality of ceramic green sheets 54 applied with internal conductors 52a having a predetermined pattern and formed via a printing method, connecting the internal conductors 52a via a hole 55 so as to form a coil, baking the coil, applying a conductor paste to a predetermined position of the element 51 and baking so as to form external electrodes 53a, 53b. 
Since the internal conductor comprising the coil is provided via a printing method, it is difficult to have a thick internal conductor 52a (in general, 20 xcexcm is said to be the upper limit). As a result, the electric resistance of the internal conductor (coil) cannot be lower than a certain level.
In order to solve this problem, an inductor, as illustrated in FIG. 8, has been introduced. This inductor comprises an internal conductor 62 prepared by forming a coil with a metal wire (such as an Ag wire) surrounded by an element 61 made from a ceramic material. The inductor also comprises external electrodes 63a, 63b provided in the element 61. However, since the ceramic element 61 and the internal conductor 62 are closely contacted, stress is generated therebetween due to the contraction difference between the ceramic 61 and the internal conductor 62 at the time of baking. This stress generates cracks in the ceramic. One skilled in the art will appreciate that stress can remain in the inductor even when cracks are not generated. Furthermore, stress can also be generated due to the contraction difference between the ceramic and the internal conductor as a result of a temperature change due to the surrounding environment or the usage condition.
The stress that remains in the inductor and the stress generated by the usage condition, as mentioned above, not only deteriorate the electric characteristics of the inductor, but may also generate cracks in the ceramic, depending upon the size of the stress. Moreover, repetition of application and release of stress also serves as the cause of crack generation in the ceramic. Crack generation leads to an increase in the leakage flux which further deteriorates the characteristics of the inductor.
The present invention seeks to overcome these deficiencies in the art by providing a inductor which reduces the risk of generating stress between a material of an element such as a ceramic, and the internal conductor and generating cracks inside the inductor chip.
An inductor according to the present invention comprises a chip element accommodating a conductor (internal conductor) and external electrodes. The internal conductor comprises a metal wire formed in a nonlinear shape. In an exemplary embodiment of the present invention, the internal conductor has a coil-like shape with portions adjacent to each other with respect to the axial direction of the coil being positioned in a substantially cylindrical gap formed in the axial direction of the coil.
Since a metal wire is used for the internal conductor, the resistance of the internal conductor can be lowered. Furthermore, since a gap is provided around the internal conductor, the stress generation between the ceramic and the internal conductor, as set forth above in association with the conventional inductor (without a gap), can be prevented. Therefore, desired characteristics can be realized with improved reliability without the risk of generating cracks inside the chip.
As indicated above, the internal conductor is formed in a nonlinear shape. xe2x80x9cNonlinearxe2x80x9d refers to various kinds of curved or wound shapes. Representative examples thereof include, but are not limited to, a zigzag (meandering) shape and a coil (spiral) shape.
The present invention is further characterized in that the chip element is formed with a magnetic ceramic or a dielectric ceramic material. Since a magnetic ceramic or a dielectric ceramic material is used as a component for the chip element, an inductor having desired characteristics can be obtained securely to realize the effects of the present invention.
The present invention is further characterized in that the internal conductor is provided by forming a wire made from a material selected from the group consisting of Ag, Cu, Ni and an alloy thereof. Since the internal conductor is provided by forming a wire made from a material selected from the group consisting of Ag, Cu, Ni and an alloy thereof, an internal conductor having a small electric resistance and a desired nonlinear shape can be formed securely to realize the effects of the present invention.
The present invention is further characterized in that the internal conductor has a coil-like shape, and portions in the metal wire comprising the internal conductor adjacent to each other with respect to the axial direction are arranged in a substantially cylindrical gap formed in the axial direction of the coil in the chip element. Since the internal conductor has a coil-like shape, a sufficient inductance can be obtained. And further, since portions of the metal wire which are adjacent to each other with respect to the axial direction are arranged in a substantially cylindrical gap formed so as to communicate in the axial direction of the coil, characteristic deterioration or crack generation in the chip caused by stress generated between the ceramic and the internal conductor can be prevented securely.
Further, since portions adjacent to each other with respect to the axial direction (i.e., coil pitch portions) in the coil-like internal conductor are integrated and accommodated in the substantially cylindrical gap, the leakage flux among the coil pitches can be reduced to improve the characteristics.
A method of producing an inductor according to the present invention comprises the steps of coating the internal conductor, comprising a nonlinear metal wire, with a covering material to be eliminated at the time of baking, placing the internal conductor coated with the covering material in a shaping mold, filling an element material around the internal conductor so as to form a compact (unbaked chip element) with the internal conductor provided at a predetermined position, and baking the unbaked chip element thereby eliminating the covering material and forming a gap around the internal conductor.
By coating the internal conductor with a covering material, and placing the same in a shaping mold, filling an element material around the internal conductor so as to form a compact (unbaked chip element) with the internal conductor provided at a predetermined position, and eliminating the covering material by baking the unbaked chip element, a gap can be formed around the internal conductor securely so that an inductor according to the present invention can be produced efficiently.
An alternative method of producing an inductor according to the present invention comprises the steps of coating the internal conductor comprising a coil-like metal wire with a covering material to be eliminated at the time of baking with portions of the metal wire adjacent to each other with respect to the axial direction of the coil integrated, placing the coil-like internal conductor coated with the covering material in a shaping mold, filling an element material around the internal conductor so as to form a compact (unbaked chip element) with the internal conductor provided at a predetermined position, and baking the unbaked chip element to eliminate the covering material so as to form a substantially cylindrical gap around the coil-like internal conductor for integrally accommodating the portions in the metal wire.
By coating the internal conductor comprising a coil-like metal wire with a covering material with portions in the metal wire adjacent to each other with respect to the axial direction of the coil integrated, placing the same in a shaping mold and filling an element material around the internal conductor so as to form a compact (unbaked chip element) with the internal conductor provided at a predetermined position and eliminating the covering material by baking the unbaked chip element, a substantially cylindrical gap for integrally accommodating the portions in the metal wire, a gap can be formed around the coil-like internal conductor securely so that an inductor according to the present invention can be produced efficiently.
The present invention is further characterized in that the covering material is selected from the group consisting of a resin material to be eliminated by decomposition or combustion at the time of baking, and a low melting point metal material to be eliminated by melting at the time of baking. By using a resin material to be eliminated by decomposition or combustion at the time of baking (such as an enamel resin), or a low melting point metal material to be eliminated by melting at the time of baking (such as solder, tin, and bismuth) as the covering material, the covering material can be eliminated securely at the time of baking so that a desired gap can be formed around the internal conductor.